The present invention relates to a system equipped with a radio frequency (RF) sensor to precisely control tracking satellite antennae which have wide angular fields of reception. This invention relates to the field of antennae, and more particularly to that of highly directional antennae, preferably for satellite applications. The invention also relates to the field of automatic beacon, or target, tracking systems.
The problem of fine or precise pointing of highly directional antennae on board satellites has only in recent years been addressed by means of RF sensors incorporated in the antennae illuminators. The principle on which all RF sensors is based is that of an RF beacon signal transmitted by a ground station, which is received on board the satellite through a device (i.e., the RF sensor) capable of detecting angular displacement of the beacon signal's direction of arrival with respect to the sensor's electrical boresight.
The most difficult problem to be solved with these systems is that of high pointing accuracy required in the presence of signal fade-outs in the satellite-to-earth path, which may reach 35 dB in communication systems which use frequency bands in the 30 GHz range. A further difficult problem to be solved is the capability of providing the device with a non-ambiguous self-acquisition angle which is wide enough to enable rapid angular detection (acquisition) or re-acquisition of the beacon signal's direction of arrival upon (i) loss of tracking due to failures in the satellite attitude control system, which permits coarse antenna attitude stabilization; (ii) a temporary emission interruption of the ground transmitter generating the beacon signal; or (iii) unexpected maneuvers of the satellite attitude control, which might cause unlocking of the auto-tracking system.
In fact, conventional systems are designed to enhance fine tracking capabilities, a peculiarity which is clearly incompatible with the capacity to re-acquire the beacon signal over a wide angular field.
As hereinafter described, the above-mentioned problems will be illustrated with reference to the description of the operation of a conventional type of tracking system. The radio frequency sensor is a device providing at its output a measureable signal, such as a voltage, proportional to the instantaneous angular displacement between the beacon's direction of arrival, expressed in satellite coordinates, and the sensor's radioelectrical boresight. The angular displacements detected by the sensor are suitably processed and used in a position servo acting on the on board antenna to reestablish the correct alignment between the sensor's boresight and the beacon's direction of arrival.
All known R.F. sensors for satellite applications are based on the well-known monopulse technique, used for Radars. A monopulse sensor is capable of directly providing an RF reference signal, called the sum signal, and difference signals having an amplitude which increases with the angular offset between the signal's direction of arrival and the sensor boresight, and a phase angle which changes sign on crossing the boresight.
As it is necessary to correct antenna position along two orthogonal axes (parallel to the roll & pitch axes of the satellite), the RF sensor provides two difference signals, each related to the axis to be controlled. These two difference signal, RF generated, are used to modulate the phase or preferably the amplitude of the sum signal which, after re-modulation, contains information relevant to the instantaneous angular displacement between the direction of arrival of the beacon signal and the sensor's boresight.
The re-modulation technique is essential to achieve weight reduction, a basic requirement on board satellites. In fact, the technique of multiplexing on one communication channel two different items of information to be processed at a later stage requires that only one modulator-demodulator unit be used.
The detection of the two components of the instantaneous angle of error along the two orthogonal axes of the sensor is usually achieved by means of a phase locked loop receiver preceded by a signal amplitude normalizer (consisting of an automatic gain control circuit) which acts on the average value of the sum signal modulated by the difference signals. The sum signal (.SIGMA.), amplitude modulated by the difference signals (.DELTA.), is coherently demodulated by mixing with the carrier, regenerated by the phase locked loop detector, and therefore cleared of the amplitude modulated component.
It will be easily understood how the monopulse sensor system is implicitly limited in connection with its angle acquisition sector, and is therefore unable to operate outside a restricted angle where the difference signal provided by the RF sensor is of smaller amplitude than the sum signal. In fact, it is only within this range that the ratio .DELTA./.SIGMA. may be linearized. In other words, the ratio is proportional to the instantaneous angular offset between the direction of arrival of the beacon signal and the sensor's electric boresight. Outside this area, there are threshold problems (the sum signal being too low, in particular in the lower part of the beam lobe and in the sidelobes region), and rapid sign inversion of the angular discrimination function takes place, due to the periodic sign changes of the phase in the sidelobes region for both the sum beam and the difference beam. As a consequence, it is impossible to utilize the monopulse sensor for angle acquisition of the beacon starting from angle offset (between beacon direction of arrival and instantaneous direction of the sensor's electrical boresight) greater than the -3 dB "sum" beamwidth. This fact is well known in Radar techniques where the target tracking with monopulse heads can take place only following target angle designation by means of a surveillance, or acquisition, Radar which in practice performs the coarse angle acquisition function of the target.
In satellite techniques, the angle acquisition or reacquisition capacity starting from significant offset of the beacon from the instantaneous direction of arrival of the sensor's boresight is important for two reasons:
(1) there is nothing on board which is capable of locking the sensor onto the initial beacon's direction of arrival;
(2) for operational systems which need to keep the number of communication system outages low, it is important to keep reacquisition times as low as possible whenever satellite attitude maneuvers or beacon station malfunctions cause the tracking system to unlock; or bring the satellite attitude outside the window for which correct tracking can be assured.
Although reinitialization of tracking may be achieved by remote control from the ground, some operational factors such as: (a) the time required to deliver commands to execute an angular sweep of the acquisition field; (b) the resulting outages; and (c) the reliability of the procedure do not render this solution very attractive.
Therefore, it is highly desirable to use a fine tracking system based on an RF sensor, which may acquire the ground beacon within an angle much larger than the -3 dB beamwidth of the antenna beam and such that it (a) may reduce, or eliminate, the requirement for interventions at the satellite control station, in particular for telecommands; (b) may be able to minimize angle acquisition/reacquisition times; and (c) may minimize the downtimes of the telecommunication system, of which the fine tracking antenna system is an integral part.